You'll have noticed that all sorts of motoring costs are dictated nowadays by how much CO2 (carbon dioxide) a car produces.
Road tax is the obvious one, but if you buy a new car you'll also have to pay an additional levy that's based on CO2 emissions; in some cases even your parking charges can be based on how much CO2 your car emits.
And if you venture into London's Congestion Charging zone you'll be exempt from paying only if your car emits less than 100g/km of CO2. So what are CO2 emissions all about, and why the focus on them? It's simple really; there's an assumption that the planet is overheating because of humans ramping up the amount of CO2 in the atmosphere.
So in a bid to get us all into more efficient cars, we're now being taxed according to how much CO2 our cars emit. If you're still struggling to see the connection, it's a simple one; when fuel is burned, carbon dioxide is produced, and the more fuel an engine burns, the greater its CO2 emissions.
So buy a car with a smaller engine (which burns less fuel) and your CO2 emissions will be reduced. The alternative to buying a car with a smaller engine is to go for one with diesel power instead of petrol.
A diesel engine works in an inherently different way from a petrol unit, which is why they're noisier and have very different driving characteristics.
More importantly though, a litre of diesel packs more energy than a litre of petrol, so you get more miles per gallon and hence the CO2 emissions (measured in grammes per kilometre) are consequently lower. Whether CO2 emissions really are the reason for any changes in the Earth's climate is another matter, and a discussion maybe for another day...